The present invention relates to a thermal-type airflow meter that detects the flow rate of intake air in an internal combustion engine.
In an electronically controlled fuel injection system of an internal combustion engine mounted in an automobile or the like, a thermal-type airflow meter capable of measuring a mass flow rate of intake air has widely been utilized. However, under a driving condition where the rotation speed of an internal combustion engine is low and the load is high, a pulsation flow accompanied by a backward flow occurs. Accordingly, in a conventional thermal-type airflow meter that cannot detect a backward flow, a large flow rate detection error occurs. In order to reduce the flow rate detection error at a time when a pulsation flow accompanied by a backward flow is measured, there has been proposed a method in which the direction of an air flow is detected and when a backward flow is detected, a flow rate signal is corrected.
For example, as represented in FIGS. 4 and 5 of Japanese Patent No. 5558599, the bridge circuit of a sensor unit is configured with an upstream heating resistor 61, a downstream heating resistor 62, an intake-air temperature detection resistor 9, fixed resistors 10, 11 and 12, and the like. As the main output signal for detecting the flow rate, the output signal Vm of the bridge circuit is utilized. However, the flowing direction cannot be determined from the output signal Vm of the bridge circuit; therefore, in addition to the output signal Vm, the intermediate electric potential between the upstream heating resistor 61 and the downstream heating resistor 62 is outputted as a backward flow signal Vd1 indicating a backward flow rate. Then, a signal obtained by subtracting the backward flow signal Vd1 from the output signal Vm of the bridge circuit is outputted as a flow rate signal Vout.
However, because in the technology disclosed in Japanese Patent No. 5558599, two signals, i.e., the output signal Vm of the bridge circuit and the backward flow signal Vd1 are utilized, the circuit configuration is complicated and upsized, thereby hindering the cost saving and the downsizing. Moreover, the variations in the electronic components that create the output signal Vm of the bridge circuit and the variations in the electronic components that create the backward flow signal Vd1 provide respective effects to the foregoing signals; thus, the accuracy of the whole signal cannot readily be maintained.
Furthermore, in the technology disclosed in Japanese Patent No. 5558599, it is not taken into consideration that because the output characteristic of the sensor unit is nonlinear and the output signal of the sensor unit has a response delay caused by a heat capacity, the output signal of the sensor unit shifts toward the positive side or the negative side; thus, there has been a problem that the accuracy of the flow rate signal is deteriorated.
Therefore, there is required a thermal-type airflow meter that reduces the number of output signals of the sensor unit and that can suppress the accuracy of flow rate detection from being deteriorated because due to the nonlinear sensor output characteristic and the response delay of the output signal, the output signal shifts toward the positive side or the negative side.